1. Technical Field
The invention relates to electrically pumped, vertical cavity, surface emitting lasers which are top emitting. Such lasers may be discrete or may be included in integrated circuits as, for example, in laser arrays or in optoelectronic circuits.
2. Description of the Prior Art
The vertical cavity, surface emitting laser structure (SEL), is emerging as a promising solution to the recognized need for inexpensive, reliable laser structures. Its two-dimensional nature, with its very small active gain region volume translates into the low lasing threshold currents which relieve the problem of heat dissipation. While the SEL is certainly of interest, as a discrete device, it is regarded by many as a significant potential breakthrough in terms of integration--both all-optical and opto-electronic. Contemplated uses include optical switching/computing, photonic interconnection, high/low power laser sources, image processing, neural networks, etc.
Reported structures have active regions based on gallium arsenide or indium gallium arsenide in the form of one or more quantum wells, or, alternatively, of bulk material. Efficient devices have placed active gain material, whether quantum wells or bulk, at positions corresponding with peak intensity values of the standing wave, with inert (non-gain) filler material elsewhere.
As the dimension of the active region is reduced in the lasing direction (to yield lowest lasing threshold values), increased cavity reflectance is needed to accommodate the correspondingly reduced per-pass gain. Needed reflectance values for each cavity end, generally 98+%, may be in the 99.4-99.9% range for low gain devices, with the latter value corresponding with the ultimate single quantum well structure. Such reflectance values generally depend upon distributed dielectric mirrors, sometimes referred to in terms of the prototypical distributed Bragg reflector (DBR). Such mirrors are made up of "periods" each including paired layers of material, transparent to and of different refractive index for the emission wavelength.
Design criteria of dielectric mirrors are well known. See, for example, M. Born and E. Wolf, "Principles of Optics", Pergammon, N.Y. (1964), p. 51 and J. P. Van der Ziel and M. Ilegems, "Applied Optics", vol. 14, no. 11, (Nov. 1975).
Reported structures for the most part are constructed by epitaxial growth on an inert substrate. A characteristic device is grown on silicon-doped n.sup.+ conductivity type gallium arsenide by molecular beam epitaxy (MBE). Initially grown material, in this instance n-doped alternating layers of e.g. aluminum arsenide and aluminum gallium arsenide, form the bottom dielectric mirror. Next comes the active region consisting of spacer (or barrier) regions embracing the active gain material. The top mirror, again alternating layers of material of differing refractive index, but this time p-doped completes the structure from the optical standpoint. Design details are adequately set forth in the literature. See K. Iga et al, "Electronics Letters", vol. 23, pp. 134-136 (1987); A. Ibaraki et al, "Japanese Journal of Applied Physics", vol. 28, pp. L667-L668 (1989). A single quantum well structure is described by Y. H. Lee et al in "Electronics Letters", vol. 25, pp. 1377-1378 (1989).
Electrically pumped structures, those of greatest interest for most uses, generally bias the active region through the mirrors, which as indicated above, are doped so as to result in the typical p-i-n configuration. Most devices have depended upon top electrodes, in-line with the lasing direction. While this is a convenient arrangement, such top electrodes are generally metallic and of such thickness as to preclude top emission, so that devices generally depend upon emission through the substrate. For (usual) GaAs substrates, the increased emission wavelength resulting from indium inclusion in the gain region (use of InGaAs active gain material) permits transmission without modification of the substrate. Use of GaAs active gain material (to which the substrate is absorbing) has been accommodated by etch-removal of in-line substrate.
A structural variation depends upon a "hybird" mirror on the p-type side of the cavity--a mirror including a conventional metallic reflecting layer supplemented by a dielectric mirror containing a lesser number of dielectric pairs. In this manner, the contribution of increased resistance, due to low hole mobility in the p-doped dielectric mirror layers is lessened while still realizing the high reflectance offered by the distributed dielectric structure.
Whether the top metal layer serves the simple function of electrode, or whether it serves as a portion of the mirror, the effect is the same--it is opaque to laser emission and so precludes top surface emission.
While the desirability of top surface emission has been recognized, for example, for use in expediently manufactured displays, structural approaches have been expensive. One approach, which relies upon lateral introduction of biasing current into the active region, uses etching, regrowth and diffusion. See M. Ogura et al., "Applied Physics Letters", vol. 51, p. 1655 (1987).